1. Field of the Invention
The present invention relates to a bipolar transistor such as a heterostructured bipolar transistor (HBT) and its manufacturing method.
2. Description of the Related Art
HBTs using an AlGaAs/GaAs heterostructre have been developed as high speed microwave or millimeterwave devices.
In order to increase the operation speed, a composition graded base layer has been introduced to reduce the transit time of carriers within the base layer.
In a first prior art HBT, the composition of Al.sub.x Ga.sub.1-x As of a base layer is graded from x=0 at the interface between a collector layer and the base layer to x=0.1 at the interface between the base layer and an emitter layer (see: JP-A-61-292365, JP-A-63-47974, JP-A-63-81977 & JP-A-4-96334). In this graded Al composition base layer, the energy band gap is gradually decreased from the emitter side to the collector side, so that a strong electric field is generated within the base layer, thus reducing the transit time of carriers within the base layer. This increases the operation speed of the HBT. This will be explained later in detail.
In the first prior art HBT, before the formation of a base electrode, the upper surface of the base layer having a high aluminum composition has to be exposed. As a result, the upper surface of the base layer is easily oxidized by the open air before the formation of the base electrode. Therefore, an ohmic contact resistance between the base electrode and the base layer is increased.
In order to reduce the above-mentioned ohmic contact resistance, the base electrode is made of metal alloy such as Pt/Ti/Pt/Au. That is, when a heat treatment is performed thereupon, the metal alloy is diffused into the base layer through the aluminum oxide therebetween, thus reducing the ohmic contact resistance therebetween. In this case, the Pt composition of the metal alloy serves as a barrier for diffusion of Ti or Au, thus obtaining a stable ohmic contact resistance (see: JP-A-4-186617).
In the first prior art HBT, however, even if the Pt composition of the base electrode serves as a diffusion barrier, it is still difficult to control a depth of diffusion of metal alloy into the base layer, since the base layer is very thin. In addition, a thermally stable ohmic contact resistance cannot be obtained.
In a second prior art HRT, a thin n-type AlGaAs emitter layer covers the entire surface of the base layer. Therefore, since the base layer is not exposed, the aluminum composition of thereof is hardly oxidized by the open air. That is, when a heat treatment is performed upon metal alloy, the metal alloy is diffused into the base layer through the thin emitter layer, thus reducing the ohmic contact resistance therebetween. This will also be explained later in detail.
Even in the second prior art HBT, it is still difficult to control a depth of diffusion of metal alloy into the base layer, since the base layer is very thin. In addition a thermally stable ohmic contact resistance cannot be obtained.
In a third prior art HBT, a base contact region is formed on the sides of the base layer by implanting ions into the base layer and the collector layer. This will also be explained later in detail.
In the third prior art HBT, however, since a high temperature heat treatment for activating implanted carriers is necessary after the ion-implantation, this heat treatment changes impurity distributions of the other semiconductor layers which deteriorates the characteristics thereof. Further, the ion-implantation damages the crystalline structure and thus increases recombination currents.
In fourth and fifth prior art HBTs (see: H. Shimawaki et al., "High-f.sub.max AlGaAs/InGaAs and AlGaAs/GaAs HBT's with p.sup.+ /p Regrown Base Contacts", IEEE Trans. on Electron Devices, Vol. 42, No. 10, pp. 1735-1744, October 1995), a p.sup.+ -GaAs base contact layer is regrown by a metal-organic molecular beam epitaxy (MOMBE) process using silicon oxide as a mask. In both cases, even if the contact area between the base layer and the base contact layer is small, a sufficiently small base resistance can be obtained (see: Y. Amamiya et al., "Lateral p.sup.+ /p Regrown Base Contacts for AlGaAs/In-GaAs HBTs with Extremely Thin Base Layers", IEEE 1996 54TH Annual Device Conference Digest, pp. 38-39, 1996). This will also be explained later in detail.
In the fourth and fifth prior art HBTs, since the base contact layers are formed without ion-implantation, the energy band gap of the base contact layers can be small, so that the ohmic contact resistance can be reduced. Also, since a high temperature heat treatment for activating carriers is unnecessary after the ion-implantation, the characteristics of the semiconductor layers are not deteriorated. Further, the damage of the crystalline structure can be avoided, and accordingly, recombination currents are not increased.
In the fourth and fifth prior art EBTs, however, when a graded Al composition layer is used as a base layer, an exposed surface of the base layer is also easily oxidized by the open air in the same way as in the first prior art EST.